The recent increase in small electrically-powered devices has increased the demand for very small electrochemical cells, usually disc-like or pellet-like in appearance, commonly referred to as button cells. Such cells, which are approximately the size of garment buttons, have diameters ranging up to about 1.0 inch and heights ranging up to about 0.60 inches. Because of their minute size, the power generating electrochemical reactions in button cells must be efficient and complete. Additionally, button cells must be manufactured with substantial precision to meet the geometric tolerances of the electrical device and to avoid leakage of corrosive electrolytes.
When the button cell includes an air cathode assembly, i.e., a combination of structural and chemical features which permit the oxygen of the air to act as the cathode, manufacturing problems are compounded. Specifically, the cell must have at least one port through which air can enter the cell. The port must be isolated from the electrolyte. The seal which isolates the electrolyte must be sufficiently tight in order to resist the internal forces tending to force electrolyte through the seal which are present when the cell is closed and increase during cell usage. These internal forces which promote leakage increase during cell usage are not only due to elevated temperature and humidity environments, but also due to increases during use of the mass of the ingredients in the cell. Accordingly, it is important to manufacture air cathode cells with the precision and tolerances which will assure tight, pressure resistant seals over the life of the cells.
Normally, metal-air button cells are constructed in two steps; the anode section and the cathode section of button cells are usually separately assembled and then permanently sealed together as the final manufacturing step. Generally, the cathode section is contained in a topless, hollow metallic container having one or more parts so as to provide air access. An air cathode sub-assembly which contains a non-metallic barrier on the surface which contacts the anodic material and a hydrophobic layer on its opposite surface, is inserted in the cathode container to create an interference fit. The anode section usually consists of a topless, hollow metallic can into which a measured amount of a metallic anode is placed. An alkaline electrolyte is then metered directly onto the surface of the anode. After the electrodes have been made, they are joined by inverting the cathode section and placing it over the open end of the anode can. The button cell is then sealed, usually by crimping the edges into a non-metallic grommet so as to hold all of the components in the desired relationship to one another and to prevent leakage of the electrolyte.
It is well documented in the art that the failure to distribute the incoming air to a substantial portion of the available surface of the air cathode restricts the current capacity of a metal-air button cell. Therefore, in order to prevent this decrease in current capacity, air chambers have been routinely provided between the metal container and the air cathode of metal-air button cells. Such chambers are provided by stamping a step in the metal container or by inserting a sealing washer between the container and air cathode.
The incorporation of a step for this purpose is taught by U.S. Pat. No. 3,897,265. In order to prevent the air cathode from pressing into this chamber, a porous diffusion member is often placed therein and held in place with a drop of adhesive. In practice, steps of this type have an approximate height of 0.008 inches to 0.015 inches and consume a significant percent (10-22%) of the total internal volume of the cell.
The use of a seal washer placed between the metal container and the air cathode so as to form an air chamber is taught and claimed in U.S. Pat. No. 4,404,266. According to those teachings, since one purpose of the washer is to improve the cell seal so as to prevent electrolyte leakage from the cell, the washer is coated with adhesive sealants such as polyamids, asphalt, etc. The thickness of the washer may vary from 0.01 inches to 0.025 inches. U.S. Pat. No. 4,404,266 teaches that it is preferable to place a porous material within the seal washer. The manufacture and handling of a relatively thin adhesive-coated washer and the placement of a porous material within it is both difficult and expensive and has yet to be demonstrated practical via commercialization.
In the manufacture of button cells, it is imperative to use as much of the available internal volume of the cell for the reactive consumable materials. Because of the size of button cells, the failure to maximize the amount of active materials consumed by the electrochemical reactions will result in cell lives that do not meet reasonable consumer expectations. An increase in cell capacity due to an increase in available internal cell volume is more dramatic in metal-air cells than in other button cell systems. In metal-air cells the cathode is catalytic and not consumable; consequently only metallic anodic material and electrolyte need to be added to provide additional capacity. In other cell systems a consumable cathodic material must be increased along with the anodic material and the electrolyte in order to increase the cell capacity.
A draw back to the use of metal-air button cells in some applications has been the time required for the activation of the cells. In storage, before first usage, a metal-air button cell normally has a seal over the air entry ports. When the cell is put into use, the seal, e.g., a piece of adhesive tape, is removed, and the cell is put into the device which it powers. The time between removal of the seal and the start of generation of power at the level required by the device should be as short as possible. Failure to activate rapidly often results in customer dissatisfaction, since the customer will perceive that the cell is dead when in fact it is not.
Because of the aforementioned increase in the demand for button cells, metal-air button cells must be manufactured inexpensively and in large volumes. Therefore, the judicious implementation of engineering principles which obviates the need for exotic, expensive or slow-acting solutions to the problems associated with metal-air button cells are required.
It is an objective of the present invention to provide a method to inexpensively manufacture large volumes of metal-air button cells by reducing the number of parts and the number of steps required in the manufacture of such metal-air cells. Another objective of the present invention is to provide a metal-air cell which maximizes the internal cell volume available for anodic material within a metal-air button cell of given external dimensions. Still another objective of the present invention provide a metal-air cell which reduces the time required for the activation of metal-air cells once they are put into service. These objectives and other subsidiary objectives are achieved by the practice of the present invention.